1. Field of the Invention
The invention pertains to the field of MOS (Metal/Oxide/Semiconductor) integrated circuits and, more particularly, to a fuse that can be used in such circuits.
2. Description of the Prior Art
The use of fuses in integrated circuits is very widespread, notably for the programming of read-only memories or in order to obtain a circuit configuration, after the breakdown (or blowing) of the fuse or fuses, that is different from the initial configuration. This is the case, for example, with integrated circuits for chip cards where the fuses, after their "breakdown" or "blowing", make it possible to block access to certain circuit zones. In general, this configuration cannot be established during the manufacture of the card, for it should be possible to have access to these zones to test the circuit and to customize it or adapt it to a specific need. It is only after this that the access to these zones must be prohibited. For this application, the security of the card is closely related to efficient breakdown of the fuses.
Other applications for fuses in integrated circuits are used, for example, in high-capacity memories for redundancy, the defective columns (or lines) being replaced after the breakdown of a corresponding fuse by supplementary columns (or lines).
There are various types of known fuses that can be used in integrated circuits, notably in programmable memories where the recording is done by the breakdown of a fuse or junction. The fuse is made in the form of a thin layer of metal consisting of an alloy of nickel and chrome or, especially, of a layer of polycrystalline silicon having excellent reliability, destroyed by the application of high power. The breakdown of the junction too calls for well-controlled conditions of breakdown with fairly high currents.
The main limitation as regards standard fuses, therefore, is the need to use high breakdown power, for example currents of 100 to 200 mA at 10 to 20 V. Now, this is incompatible with developments in the technology of integrated circuits, where the trend is towards reducing the consumption of the circuits and towards reducing the internal voltages and the breakdown voltages of the diodes or junctions.
Thus, it is more difficult to achieve the proper breakdown or blowing of a fuse in CMOS integrated circuits than in circuits of some years ago, especially because the voltage applied during the breakdown must be far lower than was the case previously.
Other problems too are poorly resolved, for example the unwanted breakdown of fuses mounted in parallel with fuses that have to be broken down: indeed, during the breakdown of a fuse, the conduction of the breakdown control transistor may cause the "stray" breakdown of a fuse which should not break down. The only way to resolve this problem consists in reducing the breakdown voltage, but this makes it more difficult to achieve the breakdown of the fuse which has to break down.
Another problem results from the breakdown of the fuse by electrostatic discharge. Indeed, this breakdown takes place under conditions which currently approach characteristics required at the inputs/outputs of the circuits. Thus, the breakdown by electrostatic discharge may take place for 500 V while the input/output voltages frequently reach 5000 V. It is therefore impossible to use fuses at the input/output pins and to achieve their breakdown.
To overcome these drawbacks, integrated circuit designers have developed and perfected electrically programmable (EPROM or EEPROM) MOS memory cells or "memory dots" for which the logic state, namely circuit open (0) or on (1), is determined by the existence or non-existence of charges that have collected at an floating (unconnected) gate. The programming of a cell such as this, namely the change from the "circuit open" state to the "circuit on" state, amounts to a shifting of the threshold of conduction by these charges that have collected at the floating gate.
A cell such as this, with two logic states, is equivalent to a fuse except as regards irreversibility. Indeed, the charges that have collected at the floating gate may be eliminated, for example by heating the circuit, by radioactivity, by electromagnetic wave effect or, again, through the compensation of these charges by charges stored in the thin oxide of the cells, between the source and drain electrodes and the floating gate. Now, the security of chip card type applications relies entirely on the fact that it should be impossible, after the breakdown of the fuse, to bring it back to the logic state existing before the breakdown.